I mean it’s written in Japanese and it's (I am fairly sure) a made up name. Seems odd to call it a "mistake" on behalf of the original writer/illustrator rather than just romanisation of Japanese being a complex thing with multiple ways to transliterate a given word and an official (but easily...
I just noticed that the covers for volumes one and two both have the protagonist’s name written on them but actually Romanised as Ruluna rather than Ruruna.